Field-responsive superparamagnetic composite nanofibers and methods of use thereof

ABSTRACT

The present invention relates to magnetic field-responsive fibers, which comprise magnetite particles and a polymeric matrix. The invention also provides methods of producing the same, in particular via electrospinning of a stably dispersed or monodispersed polymer solution, either aqueous or organic, comprising the magnetite particles, and applications thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application claims the benefit of U.S. Provisional Application No.60/575,423, filed Jun. 1, 2004, which is hereby incorporated it itsentirety.

GOVERNMENT INTEREST STATEMENT

This invention was made in whole or in part with government supportunder Contract DAAD-19-02-D0002 awarded by the United States Armythrough the Institute for Soldier Nanotechnologies, The government mayhave certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to field-responsive, composite nanofibers,methods of producing the same, and applications thereof. The presentinvention has wide application in such fields as magnetic filters,sensors, information storage, magnetic shielding, tunable composites,magnetic separation, SMART fabrics and piezomagnetic transducers.

BACKGROUND OF THE INVENTION

Magnetic composite fibers, in which magnetic nanoparticles are embeddedinto a polymeric fiber matrix, can be expected to exhibit interestingmagnetic field-dependent mechanical behavior with potential applicationsin a range of areas. Magnetic composite fibers, with particles offerromagnetic materials such as iron oxide below approximately 100 nm indiameter would, in theory, no longer exhibit the cooperative phenomenonof ferromagnetism found in the bulk, due to thermal fluctuationssufficient to reorient the magnetization direction of entire particlesand instead, might be superparamagnetic, exhibiting strong paramagneticproperties with large susceptibility. The relative magnitudes of thestiffness enhancement and fiber deformation by such fibers are expectedto increase as the diameter of the embedding polymer fiber is reduced,and therefore, to date, production of magnetic composite nanofibers(i.e. with diameters on the order of 100 nm or less) withsuperparamagnetic properties, with defined mechanical properties, hasnot been achieved.

Electrospinning is an effective method for the production of polymericnanofibers with diameters ranging from a few nanometers to a fewmicrometers. This technique has attracted interest over the last decadedue to potential applications for nanofibers numerous applications. In atypical electrospinning process, a polymer solution or melt is extrudedthrough a capillary and, in the presence of a strong electric field,deforms, resulting in ejection of a charged jet from the apex of thecone, which is accelerated toward a grounded collecting device,traveling first as a steady jet for a certain distance, and then in someinstances undergoing an electrostatically driven whipping instabilitythat bends and stretches the jet. The result of the whipping instabilityis a dramatic reduction in the diameter of the jet, typically by about 2orders of magnitude, which allows for rapid solidification of the jetthrough solvent evaporation (for solution) or cooling (for melts). Thesolid fibers are deposited on an electrically grounded collecting devicein the form of threads or as a non-woven fabric.

The incorporation of nanoparticles into nanofibers by electrospinning ananoparticle-filled polymer solution, however, necessitates dispersionstability of the nanoparticles in the polymer solution.

SUMMARY OF INVENTION

In one embodiment, this invention provides a superparamagnetic fibercomprising magnetite particles and a polymeric matrix. In oneembodiment, the fiber is a nanofiber, which in another embodiment isless than 500 nm in diameter, or in another embodiment, the nanofiberhas a diameter that ranges from 10 nm-1 μm.

In one embodiment, the matrix comprises polyethylene oxide, polyvinylalcohol or a combination threof. In another embodiment, thesuperparamagnetic fiber is magnetic field-responsive.

In another embodiment, this invention provides a field-responsive fibercomprising ferromagnetic nanoparticles and an organic polymeric matrix.According to this aspect, and in one embodiment, the fiber is ananofiber, and in another embodiment, has a diameter ranging from 10-500nm. In another embodiment, the organic polymeric matrix comprisespolymethyl methacrylate.

In another embodiment, according to this aspect of the invention, thenanoparticles are monodispersed within said polymeric matrix. In anotherembodiment, the fiber has a high saturation magnetization, ranging from250 kA/m to 2000 kA/m, In another embodiment, the fiber has a tunableNèel relaxation time which ranges from 2 milliseconds to 4 seconds, orin another embodiment, the tunable Nèel relaxation time is a function ofnanoparticle size.

In another embodiment, this invention provides a device or apparatuscomprising a field-responsive or superparamagnetic fiber of thisinvention. In one embodiment, the device or apparatus is used as afilter or a sensor, or in another embodiment, is used for informationstorage, or in another embodiment, is used for magnetic imaging, or inanother embodiment, is used for magnetic shielding. In anotherembodiment, this invention provides a fabric comprising a fiber of thisinvention, which in another embodiment, is a SMART fabric.

In another embodiment, this invention provides a method of producing afield-responsive fiber comprising magnetite particles and a polymericmatrix, the method comprising the step of electrospinning a polymersolution comprising magnetic nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 demonstrates a DLS curve for a 2.5 wt % magnetite nanoparticlesolution.

FIG. 2 is a micrograph obtained by transmission electron microscopy ofmagnetite fluid.

FIG. 3 is a plot of magnetization (M) versus magnetic field (H) for 2.5wt % as-synthesized magnetite fluid.

FIG. 4 demonstrates some representative SEM images of PEO andPEO/magnetite nanofibers: (a) PEO (1%), (b) PEO (1%)+Fe₃O₄ (3.52%), (c)PEO (2%), PEO (2%)+Fe₃O₄ (0.75%).

FIG. 5 demonstrates some representative SEM images of PVA andPVA/magnetite nanofibers: (a) PVA (7.5%), (b) PVA (7.5%)+Fe₃O₄ (0.75%),(c) PVA (7.5%)+SDS (1%), (d) PVA (7.5%)+SDS (1%)+Fe₃O₄ (0.75%).

FIG. 6 demonstrates TEM images of superparamagnetic nanofibers. (a) PEOnanofiber with 28 wt % magnetite nanoparticles. (b) PVA nanofiber with 8wt % magnetite nanoparticles.

FIG. 7 demonstrates magnetization curves of superparamagneticnanofibers: (a) PEO nanofiber with (28 wt %) magnetite nanoparticles,(b) PVA nanofiber with (8 wt %) magnetite nanoparticles.

FIG. 8 schematically depicts a tip-sample interaction during anindentation test.

FIG. 9 demonstrates indentation curves for PVA/magnetite (8 wt %)nanofiber: (a) calibration on hard surface (mica), cantilever bendingwithout indentation; (b) indentation curve on PVA/magnetite nanofiber,cantilever bending and indent; (c) indentation curve on PVA/magnetiteafter subtracting the cantilever bending.

FIG. 10 demonstrates field responsive behaviors of PVA/magnetite fabric(a) without magnetic field, (b) within a low gradient of magnetic field,(c) within a high gradient of magnetic field.

FIG. 11 demonstrates TEM images of magnetite nanoparticles. Thenanoparticles are 8, 14 and 16 nm in size, respectively (left to right).

FIG. 12 demonstrates a representative SEM image of PMMA fiber containing37 wt % of 16 nm magnetite nanoparticles.

FIG. 13 demonstrates magnetization curves of PMMA fiber containing 37 wt% of 16 nm magnetite nanoparticles.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, this invention provides a field-responsive fiber,comprising nanoparticles and a polymeric matrix.

In one embodiment, the nanoparticles range in size from about 4 nm toabout 100 nm. In one embodiment, the nanoparticles are magnetic andcomprise iron, oxides of iron, cobalt, oxides of cobalt, alloys of ironand cobalt, platinum, alloys of iron and platinum, alloys of cobalt andplatinum, manganese oxide, alloys of manganese and iron or alloys ofnickel and iron or nickel and cobalt.

In one embodiment, the nanoparticles which comprise the fibers of thisinvention can be synthesized by an organic route or, in anotherembodiment, by an aqueous route, as exemplified herein, and as will beappreciated by one skilled in the art.

In one embodiment, synthesis of the nanoparticles via an aqueous routemay comprise the steps of co-precipitating metal salts at high pH (14)in the presence of a stabilizing polymer. The stabilizing polymer hascarboxylic moieties, in some embodiments.

The present invention provides, in one embodiment a field-responsive,superparamagnetic fiber comprising magnetite particles and a polymermatrix. As exemplified herein, and representing some embodiments of theinvention, the polymer/magnetite nanofibers exhibited superparamagneticbehavior at room temperature, and deflected in the presence of anapplied magnetic field.

In one embodiment, this invention provides a superparamagnetic fibercomprising magnetite particles, which in one embodiment, refers to aniron ore that is strongly attracted by a magnet. In one embodiment“magnetite” refers to a molecule with a general formula of Fe₃O₄, whichin another embodiment, possesses a Fe²⁺ to Fe₃₊ ratio of about 1:1.5 toabout 1:2.5, or in another embodiment, about 1:2.

In another embodiment, the superparamagnetic nanofibers may compriseparticles which are chemical equivalents of magnetite, such as, forexample, and in one embodiment, (Fe,M)OFe₂0₃ where M may be, in oneembodiment, Zn, Co, Ni, Mn, or Cr. In another embodiment, the Fe²⁺ toFe³⁺ ratio includes any ratio that permits the formation of thesuperparamagnetic fibers of the present invention.

In another embodiment, the concentration of magnetite in suspsension is2.5 wt %. In another embodiment, the concentration of magnetite insuspension is 0.75 wt %. In another embodiment, the concentration ofmagnetite in suspsension is 0.75-50 wt %, or in another embodiment, theconcentration of magnetite in suspsension is 0.75-2.5 wt %, or inanother embodiment, the concentration of magnetite in suspsension is0.75-5.0 wt %, or in another embodiment, the concentration of magnetitein suspsension is 0.75-10 wt %, or in another embodiment, theconcentration of magnetite in suspsension is 0.75-15 wt %, or in anotherembodiment, the concentration of magnetite in suspsension is 0.75-20 wt%, or in another embodiment, the concentration of magnetite insuspsension is 0.75-25 wt %, or in another embodiment, the concentrationof magnetite in suspsension is 0.75-30 wt %, or in another embodiment,the concentration of magnetite in suspsension is 0.75-35 wt %, or inanother embodiment, the concentration of magnetite in suspsension is0.75-40 wt %, or in another embodiment, the concentration of magnetitein suspsension is 0.75-50 wt %, or in another embodiment, theconcentration of magnetite in suspsension is 2.5-10 wt %, or in anotherembodiment, the concentration of magnetite in suspsension is 2.5-20 wt%, or in another embodiment, the concentration of magnetite insuspsension is 2.5-25 wt %, or in another embodiment, the concentrationof magnetite in suspsension is 2.5-30 wt %, or in another embodiment,the concentration of magnetite in suspsension is 2.5-40 wt %, or inanother embodiment, the concentration of magnetite in suspsension is2.5-50 wt %, or in another embodiment, the concentration of magnetite insuspsension is 10-20 wt %, or in another embodiment, the concentrationof magnetite in suspsension is 10-30 wt %, or in another embodiment, theconcentration of magnetite in suspsension is 10-40 wt %, or in anotherembodiment, the concentration of magnetite in suspsension is 10-50 wt

The superparamagnetic fibers of this invention comprise, in oneembodiment, magnetite particles and a polymer matrix. In one embodiment,the polymers comprising the polymer matrix of this invention may becopolymers. In another embodiment, the polymers may be homo- or, inanother embodiment heteropolymers. In another embodiment, the polymersmay be synthetic, or, in another embodiment, natural polymers. Inanother embodiment, the polymers may be water-soluble. In anotherembodiment, the polymers comprising the polymer matrix of this inventionmay be free radical random copolymers, or, in another embodiment, graftcopolymers. In one embodiment, the polymers may comprisepolysaccharides, oligosaccharides, proteins, peptides or nucleic acids.It is to be understood that any polymers, which may be utilized toproduce a superparamagnetic fiber of this invention, such as, anymaterial that may be electrospun into a fiber, including, in otherembodiments, any natural or synthetic polymer, are to be considered aspart of this invention.

In one embodiment, the choice of polymer utilized may be a function ofthe particles employed. In one embodiment, the polymer may comprisepolyacrylic acid, polystyrene sulfonic acid, polyvinyl sulfonic acid,polyethylene oxide polypropylene oxide, polyvinyl alcohol, or acombination thereof.

In another embodiment, the polymer comprises a surfactant, apolyethylene glycol, a lignosulfonate, a polyacrylamide or a biopolymer.In another embodiment, the biopolymer may comprise polypeptides,cellulose and its derivatives such as hydroxyethyl cellulose andcarboxymethyl cellulose, alginate, chitosan, lipid, dextan, starch,gellan gum or other polysaccharides, or a combination thereof.

In another embodiment, the polymer comprises polyethylene oxide at aconcentration of 2-4 wt %. In another embodiment, the polymer comprisespolyacrylic acid, at a concentration of 6.5-15 wt %. In anotherembodiment, the polymeric matrix may comprise poyacrylic acid and SDS.In another embodiment, the SDS or other similar ionic surfactant may beat a concentration of 0.5-10 wt %

According to this aspect of the invention, and in another embodiment,the molecular weight of the polyacrylic acid may be 5,000 Da, or inanother embodiment, 5,000-20,000 Da. In one embodiment, the PAA andJeffamine are used during nanoparticle synthesis to form a “corona” onthe magnetite particles that allows them to be suspended in solution andstabilizes them against aggregation. In another embodiment, magnetiteparticles may be similarly prepared, via methods known to one in theart, to form stable suspensions in solution.

In another embodiment, this invention provides a field-responsive fibercomprising ferromagnetic nanoparticles and an organic polymeric matrix.In another embodiment, according to this aspect of the invention, thenanoparticles are monodispersed within said polymeric matrix.

The ferromagnetic nanoparticles can exhibit a spontaneous magnetization,and may comprise Fe, Co, Ni, Gd, Dy, MnAs, MnBi, MnSb, CrO₂, MnOFe₂O₃,FeOFe₂O₃, NiOFe₂O₃, CuOFe₂O₃, MgOFe₂O₃, EuO, Y₃Fe₅O_(12.)

In one embodiment, the organic route synthesis uses organometallicprecursors decomposed at a high temperature, which in one embodiment, isat a range of between about 200-400° C., in a high boiling point organicliquid. In one embodiment, the organic liquide is a benzyl ether, or inanother embodiment, a phenyl ether, or in another embodiment, octanol,or others, as will be appreciated by one skilled in the art. In oneembodiment, the process is conducted in the presence of stabilizers likeoleic acid and olylamine. According to this aspect of the invention, andin one embodiment, the method for preparing the nanoparticles via anorganic route produces monodisperse magnetic nanoparticles.

In one embodiment, the nanoparticles thus prepared result in particlesrelatively small in size. In one embodiment, seed mediated growth can beused to synthesize larger nanoparticles, in which the smallernanoparticles synthesized can be used as seeds in a subsequent synthesisof larger-sized nanoparticles.

According to this aspect, and in one embodiment, the functionalizationof the nanoparticles with an organic surface coating providesnanoparticles compatible with organic solvents. In one embodiment, theorganic route synthesis permits a wide range of particle compositions,including those with larger intrinsic magnetic moments, which, inanother embodiment, provide a longer Néel relaxation time. This wasexemplified herein, in Example 6 via SQUID test, where the remnantmagnetization at zero field for the 16 nm particles indicatedferromagnetic behavior, rather than the superparamagnetic behaviorexhibited by the smaller particles synthesized via aqueous route.

In one embodiment, a longer Néel relaxation time allows for changes inmechanical properties under a uniform applied field at conventionalrates of deformation. Due to coupling of the particle magnetic momentwith the applied field, deformation of the magnetic fibers requiresadditional work, resulting in increased stiffness and lower strain,compared to the equivalent nonmagnetic fibers at equal deformationenergy.

According to this aspect of the invention, and in one embodiment, thefiber has a high saturation magnetization, ranging from 250 kA/m to 2000kA/m. In another embodiment, the fiber has a tunable Nèel relaxationtime which ranges from 2 milliseconds to 4 seconds, or in anotherembodiment, the tunable Nèel relaxation time is a function ofnanoparticle size.

In one embodiment, the fibers of this invention are formed from asolution whose concentration of polymer may range from 0.5-40 wt %, orin another embodiment, the concentration ranges from 2-20 wt %, or inanother embodiment, the concentration ranges from 5-15 wt %, or inanother embodiment, the concentration ranges from 6.5-15 wt %. In oneembodiment, the polymer concentration will be a function of thechemistry, molecular weight, or combination thereof of the polymerand/or solvent used.

In one embodiment, the fibers of this invention are nanofibers. In oneembodiment, the nanofiber is less than 500 nm in diameter. In anotherembodiment, the nanofiber diameter ranges from 10nm-1 μm. In oneembodiment, the nanofiber diameter ranges from 65-100 nm, or, in anotherembodiment, the nanofiber diameter ranges from 65-200 nm, or, in anotherembodiment, the nanofiber diameter ranges from 65-300 nm, or, in anotherembodiment, the nanofiber diameter ranges from 65-400 nm, or, in anotherembodiment, the nanofiber diameter ranges from 65-200 nn, or, in anotherembodiment, the nanofiber diameter ranges from 100-200 nm, or, inanother embodiment, the nanofiber diameter ranges from 150-250 nm, or,in another embodiment, the nanofiber diameter ranges from 100-300 nm,or, in another embodiment, the nanofiber diameter ranges from 100-400nm, or, in another embodiment, the nanofiber diameter ranges from100-250 nm, or, in another embodiment, the nanofiber diameter rangesfrom 200-300 nm, or, in another embodiment, the nanofiber diameterranges from 200-350 nm, or, in another embodiment, the nanofiberdiameter ranges from 200-450 nm, or, in another embodiment, thenanofiber diameter ranges from 10-200 nm, or, in another embodiment, thenanofiber diameter ranges from 75-500 nm. In another embodiment, thenanofiber diameter ranges from 10 nm-10 μm. In another embodiment, thenanofiber has a diameter that is less than 10 nm.

In one embodiment, the magnetite nanoparticles produced via aqueousroute are stably dispersed within the polymeric matrix. In oneembodiment, the term “stably dispersed” or “stabilized” or“stabilization” refers to the stability of the resulting polymersolution or matrix, following their production. In one embodiment, theterms “stably dispersed” or “stabilized” or “stabilization” refer to thefact that the magnetite particles do not aggregate or “settle out” insolution, or in another embodiment, are readily dispersed, such as, viavortexing in solution. In another embodiment, the magnetitenanoparticles dispersed within the polymeric matrix are“colloidally-stable”. In one embodiment, the term “colloidally-stable”refers to refer to the fact that the magnetite particles do notaggregate or “settle out” in solution, or in another embodiment, form ahomogeneous solution, wherein the particles, in another embodiment,cannot be separated by ordinary filtration or centrifugation.

In one embodiment, the magnetite nanoparticles produced via organicroute are monodispersed within the polymeric matrix. In one embodiment,the term “monodispersed” refers to a relative average particle size,with a coefficient of variance of less than 10.

In another embodiment, the magnetite particles do not change in theirchemical composition over a particular period of time.

In another embodiment, the fibers of this invention may further comprisea targeting moiety. The term “targeting moiety”, in one embodiment,refers to a specificity conferred to the moiety, which results inattachment of the moiety to a cognate partner, or, in anotherembodiment, an ability to specifically “target” the moiety to a desiredcognate partner molecule. The targeting moiety may, in one embodiment,facilitate attachment of the the fibers, through the targeting moiety,to a molecule of interest, such as a protein or glycoprotein, in oneembodiment, or, in another embodiment, to a nucleic acid of interest, orin another embodiment, to a cellular fraction of interest. Such aproperty may be additionally useful, in another embodiment, inapplication of the superparamagnetic fibers of this invention infiltration or magnetic separation.

In one embodiment, the targeting moiety enhances attachment to amolecule in low abundance, which is of interest. In another embodiment,the targeting moiety enhances attachment following supply of an energysource, such as a UV light source. In one embodiment, the targetingmoiety is chemically attached to the polymers via a chemicalcross-linking group, or in another embodiment, forms a stableassociation with a polymer, or, in another embodiment, forms anassociation with the polymer, yet readily dissociates following changesin solution conditions, such as, for example, salt concentration or pH.

In one embodiment, the targeting moiety may be an antibody, whichspecifically recognizes a molecule of interest, such as a protein ornucleic acid. In another embodiment, the antibody may specificallyrecognize a reporter molecule attached to a molecule of interest. Inanother embodiment, the targeting moiety may be an antibody fragment,Protein A, Protein G, biotin, avidin, streptavidin, a metal ion chelate,an enzyme cofactor, or a nucleic acid. In another embodiment, thetargeting moiety may be a receptor, which binds to a cognate ligand ofinterest, or associated with a cell or molecule of interest, or inanother embodiment, the targeting moiety may be a ligand which is usedto “fish out” a cell via interaction with its cognate receptor.

It is to be understood that any component of interest, such as a cell,or component thereof, wherein its separation from other materials isdesired, which is amenable to the present technology is to be consideredas part of this invention.

The fibers of this invention are magnetic field-responsive, such as wasdemonstrated, for example with superparamagnetic fibers of thisinvention, in Example 5. In one embodiment, the superparamagnetic fiberproperty, or ferromagnetic property of magnetic field-responsiveness isexploited in a variety of applications, as is discussed furtherhereinbelow.

In one embodiment, the term “magnetic field-responsive” refers to theproperty of the fibers of this invention to exhibit a structuralmodification, in response to the application of an external magneticfield. In one embodiment, such responsiveness is completely reversible,or, in another embodiment, mostly, or in another embodiment, partlyreversible. In another embodiment, magnetic field responsiveness resultsin a high stiffness exhibited in the fibers of this invention. Inanother embodiment, it results in deformation or change of shape, suchas, for example, exhibited in the superparamagnetic fibers. Such changesin stiffness and/or deformation may be rate-sensitive, in anotherembodiment.

In one embodiment of this invention, the fibers of this invention areproduced via an electrospinning technique. Preparation ofsuperparamagnetic or ferromagnetic polymeric nanofibers viaelectrospinning is exemplified hereinbelow. In one embodiment, forpreparation of the former, electrospinning is conducted as describedherein, on colloidally-stable suspensions of magnetite nanoparticles inpolyethylene oxide and polyvinyl alcohol solutions. In some instances,the magnetite nanoparticles were aligned in columns parallel to thefiber axis direction within the fiber by the electrospinning process. Inanother embodiment, for the preparation of the ferromagnetic nanofibers,the electrospinning is conducted on monodispersed magnetitenanoparticles of size up to 16 nm, in THF.

In one embodiment, this invention provides a method of producing afield-responsive fiber comprising magnetite particles and a polymericmatrix, the method comprising the step of electrospinning a polymersolution comprising stably dispersed superparamagnetic magnetiteparticles, or monodispersed magnetite particles.

In one embodiment, the method of producing a field-responsive fiber ofthis invention via electrospinning comprises the step of preparingdesired concentrations of polymer/magnetite nanoparticle solutions, inwhich the nanoparticles are dispersed.

For preparation of superparamagnetic fibers of this invention, in oneembodiment such dispersions are prepared by adding the desired amount ofpolymer solution directly to a magnetite nanoparticle aqueous solution,which may be accompanied by, in another embodiment, vigorous stirring,which may be accomplished, in another embodiment, for a period of timeof at least 24 hours at room temperature.

In one embodiment, aqueous solutions of magnetite nanoparticles may beprepared as follows: aqueous solutions containing iron (III) chloridehexahydrate, iron (II) chloride tetrahydrate, and graft copolymer may bedissolved in deoxygenated water, where the graft copolymer may compriseJeffamine and polyacrylic acid.

In one embodiment, the particles are co-precipitated in the presence ofa stabilizing polymer as described hereinbabove, which, in anotherembodiment, attaches to the particle surfaces and confers stericstabilization to the particle dispersion in the polymer solution. Inanother embodiment, the magnetic fluid thus formed, comprising anaqueous solution of stably dispersed magnetite particles in polymer, maybe subjected to centrifugation and/or filtration, which, in anotherembodiment, may serve remove excess polymer and/or salts.

For preparation of ferromagnetic fibers of this invention, in oneembodiment such dispersions are prepared by by adding the desired amountof polymer solution directly to a magnetite nanoparticle organicsolution, which may be accompanied by, in another embodiment, vigorousstirring, which may be accomplished, in another embodiment, for a periodof time of at least 24 hours at room temperature. In one embodiment, thesolution will comprise THF and DMF.

In one embodiment, electrospinning may be conducted with the aid of anysuitable apparatus as will be known to one skilled in the art. In oneembodiment, a parallel-plate electrospinning apparatus may be used, suchas that described by Shin et al [Shin M., Hohman M. M., Brenner M. P.,and Rutledge G. C., Appl. Phys. Lett. 2001; 78:1149-1151] and/orFridrikh et al [Fridrikh S. V., Yu J. H., Brenner M. P., and Rutledge G.C. Phys. Rev. Lett. 2003; 90:144502].

In one embodiment, electrospinning is conducted with two 10 cm indiameter aluminum disks, arranged parallel to each other, at a distanceof up to 30 cm. In one embodiment, the electrical voltage, solution flowrate and distance between the two parallel plates are adjusted to obtaina stable jet.

In one embodiment, the methods of this invention producesuperparamagnetic fibers in which magnetite particles line up within thefibers in parallel to the fiber axis direction.

In one embodiment, the method employs magnetite nanoparticles dispersedas stable suspensions in PEO solutions, for producing superparamagneticfibers. Such solutions comprising the nanoparticles will, in anotherembodiment, exhibit increased conductivity, such as, for example, thatshown in Table 1.

In one embodiment, the methods of this invention will employelectrospinning wherein the parameters comprise a flow rate of between0.005 to 0.5 ml/minute, or in another embodiment, 0.01 to 0.03ml/minute. In another embodiment, the electrospinning parameters maycomprise a polymer/particle solution viscosity of 0.1 to 20 (Pa·s), ormore.

In one embodiment, the polymer is PVA, at a concentration ranging from6.5-15 wt %, and the viscosity ranges from 0.05-20 (Pa·s), or more. Inanother embodiment, the solution further comprises SDS, which increasesthe viscosity. According to this aspect of the invention, and in oneembodiment, a solution comprising 1 wt % SDS, exhibits a viscosity of0.6-21 (Pa·s). In one embodiment, the addition of SDS increasesviscosity and conductivity of the solution, however does not affectsurface tension properties. In another embodiment, the inclusion of SDSin the polymer solution decreases the diameter of the fiber formed.

In another embodiment, flow rate, viscosity, concentration, orcombination thereof may vary, and be any value which enableselectrospinning of the solution to produce a superparamagnetic fiber, orferromagnetic fiber of this invention.

It is to be understood that any embodiment listed herein in reference tothe fibers of this invention, may characterize the fibers as obtained bythe methods provided herein, and is to be considered as part of thisinvention.

In one embodiment, in regard to the methods of this invention producingsuperparamagnetic fibers of this invention, which comprise a PEOpolymer, the PEO polymer used is at a concentration of between 1% and 3%by weight, and in another embodiment, solutions comprising the same havea conductivity of at least 1000 μS/cm. In another embodiment thesolution may have a conductivity ranging <1 microSeimen/cm up to 1300microSeimens/cm, or greater.

In another embodiment, according to this aspect of the invention, themethods of this invention emloy the use of a polymer solution comprisingpolyvinyl alcohol, which in another embodiment, is at a concentration ofbetween 6.5% and 15% by weight, or in another embodiment, comprises SDS,which in another embodiment, exhibits a conductivity of at least 1000μS/cm.

In another embodiment, this invention provides a device or apparatuscomprising the field-responsive fibers of this invention. It is to beunderstood that such a device or apparatus may comprise any embodimentof any fiber of this invention.

In one embodiment, the device or apparatus is used as a filter. In oneembodiment, the superparamagnetic fibers of this invention are soarranged in a matrix as to form an impermeable barrier, when not underthe influence of an external magnetic field. According to this aspect ofthe invention, and in one embodiment, upon exposure to an externalmagnetic field the fibers deform in a given orientation, such that gapsare introduced within the matrix, thereby introducing permeability. Inanother embodiment, the filter is suited for use in magnetoseparation.According to this aspect, and in another embodiment, magnetic particlessuspended in a liquid are adsorbed and separated off with use of a layerof filter medium comprising the superparamagnetic fibers of thisinvention. In one embodiment, application of an external magnetic fieldmay result in deformation of the superparamagnetic fibers comprising thefilter, facilitating passage of debris, and other non-desiredcomponents, while materials adhered to magnetic particles remainattached to the filter. These are some, non-limiting applications of thesuperparamagnetic fibers as filters, however many other applicationswill be appreciated by one skilled in the art, and are to be consideredas embodiments of this invention.

In one embodiment, the device or apparatus is used as a sensor. In oneembodiment, the sensor will take advantage of the deformation of thesuperparamagnetic fibers in response to an externally applied magneticfield. In one embodiment, such sensors may be utilized for remotelysensing the alternating currants (AC) in a set of substantially parallelconductors, from the magnetic fields generated by these currents in thevicinity of the conductors. It is to be understood that any applicationwherein detection of the presence of a magnetic field is desired, and asensor comprising the superparamagnetic fibers of this invention may beutilized is to be considered as part of this invention. In anotherembodiment, as a tunable reinforcement material, the superparamagneticfibers of this invention, or a fabric comprising the same, could beincluded in a composite or multilayer construction such that thecomposite on multilayer is made stiffer when a magnetic field is turnedon, such stiffness increase being higher when the field is higher and/orwhen the rate of deformation is faster. In another embodiment, as apiezomagnetic transducer, deformation of the fabric may result in ameasureable magnetic field to sense or actuate other components. Inanother embodiment, introduction of a magnetic field may result indeflection or deformation of the piezomagnetic fiber, fabric ormultilayer material.

In another embodiment, the superparamagnetic fibers of this inventionmay be used for information storage. In one embodiment, thesuperparamagnetic fibers of this invention are used in a magneticstorage medium containing a magnetic material. The magnetic material maybe any magnetic material, which can store data by the alignment of thedirections of the spins in the material. In one aspect, the eachmagnetite particle within the superparamagnetic fibers of this inventionmay be adapted to store one bit of data.

In another embodiment, the superparamagnetic fibers of this inventionmay be used for magnetic imaging. In one embodiment, an image may beformed by applying an external magnetic field to selected regions of acomposite medium comprising the superparamagnetic fibers of thisinvention. In response to the applied field, deformation of the fibersoccurs, producing a latent image, which may, in another embodiment, bedeveloped by exposure to magnetic fluid or powders. The image may beerased, in another embodiment, by removal of the magnetic field, or inanother embodiment, by exposure to an AC demagnetizing field or a DCsweep magnet.

In one embodiment, the superparamagnetic fibers are used in magneticshielding. In another embodiment, this invention provides afabric-comprising the superparamagnetic fibers.

In many situations, such as, for example, in military shelters,protection against electro-magnetic interference signals is desired. Inone embodiment, shielding of electronic equipment is desired, as theequipment would malfunction if subjected to electro-magnetic waves. Inanother embodiment, when certain types of electronic equipment are used,it is desirable to have shielding, which prevents detection of thelocation from which the signals are generated. In one embodiment, suchshielding may be provided by the superparamagnetic fibers of thisinvention.

In one embodiment, a fabric comprising the superparamagnetic fibers, orferromagnetic fibers of this invention may be thus utilized.

In another embodiment, a fabric comprising the superparamagnetic orferromagnetic fibers of this invention, may further comprise additionalmaterials which do not materially affect their properties, such as, forexample, pigments, antioxidants, stabilizers, surfactants, and others aswill be appreciated by one skilled in the art. In one embodiment, afabric of this invention may be a SMART fabric, in which stiffness canbe controlled by use of an external magnetic field, and which absorbimpact at pre-determined rates. These properties are in addition to thefield-dependent deflection behavior demonstrated previously.

In one embodiment, the surface coating of nanoparticles will becompatible (for example, hydrophilic or hydrophobic) with the polymersolution in which they are electrospun (magnetic nanoparticles shoulddisperse uniformly in the polymeric solution).

In one embodiment, the magnetic nanoparticles will have a highsaturation magnetization and tunable Nèel relaxation time (with size).The magnetization of the material may vary, in some embodiments, from250 kA/m to 2000 kA/m. The Nèel relaxation time may vary, in someembodiments, from milliseconds to seconds, depending on the size ofnanoparticles.

The following examples are presented in order to more fully illustratesome embodiments of the invention. They should, in no way be construed,however, as limiting the scope of the invention.

EXAMPLES Materials and Methods

Materials:

Poly(ethylene oxide) (PEO, Mv 2,000,000), poly(vinyl alcohol) (PVA,87%-89% hydrolyzed, Mw: 85 k-146 k)and dodecyl sulfate, sodium salt(98%)(SDS) were obtained from Aldrich and used for makingelectrospinnable solution. Poly(acrylic acid) (PAA; 50 wt % in water,Mw=5000), iron(III) chloride hexahydrate (97%), iron(II) chloridetetrahydrate (99%), ammonium hydroxide (28 wt % in water), dimethylformamide (DMF) and dicyclohexylcarbodiimide (CDI) were obtained fromAldrich (Milwaukee, Wis.) and used for synthesizing nanoparticles.Jeffamine XTJ-234 (PEO/PPO-NH2, EO:PO=6.1:1, Mw=3000) is anamine-terminated random copolymer of ethylene oxide (EO) and propyleneoxide (PO) repeat units with 6.1 EO units per PO unit. It was donated byHuntsman Corp. (Houston, Tex.) and has characteristics similar to thatof pure PEO.

Preparation of Nanoparticles:

The graft copolymer was prepared by reacting the Jeffamine with thecarboxyls on PAA via amidation chemistry as described [Moeser G. D., K.A. R., Green W. H., Laibinis P. E., and Hatton T. A. Ind. Eng. Chem.Res. 2002; 41:4739-4749]. Only a small percentage (16%) of carboxylgroups were grafted with Jeffamine since free carboxyl groups arerequired for chelation with surface iron atoms and stabilization of themagnetite nanoparticles.

In a typical procedure for the synthesis of the magnetite nanoparticles[Moeser, supra], an aqueous solution containing 2.35 g of iron (III)chloride hexahydrate, 0.86 g of iron (II) chloride tetrahydrate, and 1 gof graft copolymer was prepared by dissolving the reagents in 40 mL ofdeoxygenated water. Deoxygenation was achieved by bubbling with nitrogenunder vigorous stirring for 30 min before reaction. The aqueous solutionwas heated to 80° C., and 5 ml of 28 wt % of ammonium hydroxide wasadded to precipitate iron oxide in the form of magnetite. The growth ofspherical nanoparticles was arrested by the polymer in the solution,which caps the magnetite nanoparticles as soon as they form andstabilizes them sterically against aggregation. The resulting mixturewas then aged for 30 min at 80° C. This procedure produces 1 g ofmagnetite in 40 mL of water, which is equivalent to a 2.5 wt %suspension of magnetite. The final magnetic fluid was washed in acentrifuge with an ultrafilter (Millipore, Centricon Plus 80, MWCO100,000) to remove excess polymer and salts.

Preparations of Spinning Solutions:

PEO solutions ranging from 1% to 3% by weight were prepared by directlyadding the PEO polymer to distilled water. The solutions were stirredvigorously for at least 24 h at room temperature in order to obtainhomogeneous solutions. PVA solutions ranging from 6.5% to 15% by weightwere prepared by directly adding the polymer into distilled water, withvigorous stirring for at least 3-4 hours at 70° C.

Various concentrations of PEO/magnetite nanoparticle dispersions wereprepared by adding the desired amount of PEO solution directly to thenanoparticle aqueous solution prepared as described above, with vigorousstirring for at least 24 h at room temperature. A range of PVA/magnetitenanoparticle suspensions was prepared similarly, and then mixed using avortex mixer (VWR Scientific Products) for at least ten minutes beforespinning, as the particle suspension was not particularly stable againstaggregation and settling.

Electrospinning:

The parallel-plate electrospinning apparatus used was similar to thatdescribed by Shin et al [Appl. Phys. Lett. 2001; 78:1149-1151] andFridrikh et al [Phys. Rev. Lett. 2003; 90:144502]. Briefly, two aluminumdisks with diameters of 10 cm were arranged parallel at a distance of upto 30 cm apart. The fluid was pumped at a constant flow rate by asyringe pump (Harvard Apparatus PHD 2000) to a stainless steel capillarywith inner diameter 1 mm located in the center of the upper disk. Anelectrical potential was applied to the upper disk by a high voltagepower supply (Gamma High Voltage Research ES-30P). Current was measuredby a Digital multimeter (Fluke85 III) as the voltage drop across a 1.0MW resistor between the lower disk and ground. The electrical voltage,solution flow rate and distance between the two parallel plates wereadjusted to obtain a stable jet.

Measurement and Characterization of Composite Nanofibers:

Viscosity was measured on an AR-7000 Rheometer (TA Instruments) at 25°C. A. Kruss 10 tensiometer was used to determine surface tensions, whileconductivity was measured using a Cole Parmer 19820 conductivity meter.

Dynamic Light Scattering (DLS):

Dynamic Light Scattering (DLS) was performed to determine thehydrodynamic diameters of the coated nanoparticles using a Brookhaven BI200-SM system at a fixed angle of 900. The autocorrelation function wasfitted with an exponential curve to obtain the diffusion coefficient,which was then used to calculate the hydrodynamic diameter via theStokes-Einstein equation.

Scanning Electron Microscopy (SEM):

Specimens for Scanning Electron Microscopy (SEM) were prepared by directdeposition of the electrospun nanofibers on an aluminum foil andsputter-coating with gold using a Desk II cold sputter/etch unit (DentonVacuum LLC, NJ). The images of the electrospun fiber were obtained usinga JEOL-6060SEM (JEOL Ltd, Japan), and the fiber diameters weredetermined using AnalySIS image processing software (Soft Imaging SystemCorp., Lakewood, USA) by measuring 20 randomly selected fibers for eachsample.

Transmission Electron Microscopy (TEM):

For Transmission Electron Microscopy (TEM), a dilute magnetitenanoparticle solution was dried on a carbon grid and visualized underthe JEOL JEM200 CX TEM microscope (JEOL Ltd, Japan) to estimate the coresizes of the particles. The electrospun nanofibers were directlydeposited onto a copper grid for TEM analysis.

Super Conducting Quantum Interference Device (SQUID) Test:

The SQUID test was conducted using an MPMS XL magnetometer (QuantumDesign Inc., San Diego) for both PVA/magnetite and PEO/magnetitenanofibers. The sample was scanned in 40-50 equal increments with anapplied magnetic field ranging from approximately −0.6 Tesla to 0.6Tesla.

Nanoindentation:

Nanoindentation experiments were performed using a Nanoscope IV,Dimension™ 3100 AFM (Digital Instrument, Santa Barbara) with a RTESPsingle-beam silicon probe (Digital Instrument) (fR=280-361 kHz, k=30-40N/m). All the nanofibers were conditioned in a vacuum oven for at leasttwo days before experiments, at room temperature for PEO andPEO/magnetite nanofibers and at 60° C. for PVA and PVA/magnetitenanofibers, respectively. During these AFM indentation tests, PVA andPVA/magnetite nanofibers were treated as a group, as were the PEO andPEO/magnetite nanofibers. Within each group, the maximum indentationforce, Pmax, was the same. Pmax for the PVA group was twice that for thePEO group. Within each group, mica and a flat reference sample of epoxywere indented using the same probe and parameters. The mica has anelastic modulus of ˜171 GPa and Poisson ratio of ˜0.3 [27-29]. Theelastic modulus of the reference epoxy sample was determinedindependently using a Triboindenter with Berkovich-type indentation tip(Hysitron Inc., Minneapolis). For each sample, at least 20 individualforce curves were obtained.

Field Responsive Testing:

A rectangular strip (length×width×thickness=1.8×0.555×0.004 cm) ofelectrospun nonwoven mat was placed on the surface of a table, with oneend fixed by tape onto the table surface, A permanent laboratory magnetwith a rectangular cross-section (1.8×0.6 cm) was suspended somedistance away above the mat, and the response behavior of the nonwovenmat to the laboratory magnet was recorded by a digital camera.

Example 1 Synthesis of Composite Nanofibers Containing MagnetiteNanoparticles

The size distribution of the magnetite nanoparticles was determined byDLS (FIG. 1), and corresponded to an average hydrodynamic diameter of 25nm. An analysis of TEM images of the as-synthesized magnetitenanoparticles (FIG. 2) indicated an average core size, assuming a lognormal distribution, of 7.5±2.9 nm. Only the magnetite cores werevisible in TEM measurements, as the polymer coatings were of lowcontrast, and could not be discerned in these images. The differencebetween the average hydrodynamic diameter and core size yielded athickness of about 9 nm for the polymer shell.

The dependence of the magnetization, M, of the magnetite fluid on theapplied magnetic field in the SQUID tests is shown in FIG. 3. Themagnetite nanoparticle suspension exhibited superparamagnetic behaviorin that there was zero remnant magnetization at zero applied field. Thesaturation magnetization was approximately 0.5-0.7 Tesla.

The magnetite nanoparticles were readily dispersed as stable suspensionsin PEO solutions, increasing their conductivity dramatically, as shownin Table 1. TABLE 1 Solution properties and electrospinning processingparameters of some representative nanofibers Flow Fiber ConductivityViscosity Voltage Rate Distance Current Diameter Composite (μS/cm) (Pa ·s) (kV) (ml/min) (cm) (nA) (nm) PEO 107.9 1.565 9.0 0.010 25 83 390 ± 40(2 wt %) Fe₃O₄ (0.75%) 1277 1.506 9.0 0.020 25 353 400 ± 80 PVA 3510.2905 29.0 0.010 25 412 170 ± 40 (7.5%) Fe₃O₄ (0.75%) 1372 0.3926 29.00.010 25 1534 320 ± 40 Fe₃O₄ (0.75%) + SDS (1%) 2740 1.941 28.5 0.016 251050 140 ± 30

The preferred electrospinning parameters (Table 1) were almost identicalfor PEO and PEO/magnetite solutions. Some representative SEM pictures ofelectrospun nanofibers of PEO and PEO/magnetite are shown in FIG. 4. Atlow PEO concentration (1 wt %) in the absence of magnetitenanoparticles, the fibers adopted a bead-on-string morphology (FIG. 4(a)), while fibers with uniform diameters were obtained when 3.52 wt %magnetite nanoparticles were added to the spin solution (FIG. 4(b)). Athigher concentrations (2-3 wt %) of PEO, uniform fiber morphologies wereobtained for both PEO and PEO/magnetite solutions with little change infiber diameters on the addition of magnetite nanoparticles, as shown inFIGS. 4 (c) and (d).

The magnetite nanoparticles were easily dispersed in PVA solutions, aswell, but these solutions were not as stable as in the case of PEO, andthe magnetite nanoparticles settled overnight. The settling of thesuspension on standing may have been due to PEO-based shells around thenanoparticles not being as compatible with the PVA in solution as theywere with PEO solutions. The PVA suspensions were easily homogenized,however, using a Vortex mixer for 10 minutes immediately beforeelectrospinning. The presence of the magnetite nanoparticles in the PVAsolutions increased the conductivity of these solutions (Table 1).Again, there was little change in the preferred electrospinningprocessing parameters for PVA solutions when magnetite nanoparticleswere added (Table 1). SEM pictures of electrospun nanofibers using PVAand PVA/magnetite solutions (FIG. 5) show that the magnetitenanoparticles lead to increased fiber diameters, but that addition ofSDS to the solution counteracted this effect, as SDS generally reducesthe fiber diameters (Table 1). The results of a detailed study of theeffect of SDS on the PVA fiber morphology are shown in Table 2. TABLE 2Effect of SDS On PVA Solution Properties and PVA Fiber MorphologyConductivity Viscosity Surface Tension Fiber Diameter (S/cm) (Pa · s)(mN/m) (nm) PVA SDS SDS SDS SDS (wt %) No SDS (1 wt %) No SDS (1 wt %)No SDS (1 wt %) No SDS (1 wt %) 6.5%  329 1223 0.1187 0.6456 41.58 42.04Beaded  96.04 ± 17.13  8% 315 1187 0.2905 1.375 39.5 42.42 146.67 ±10.90  73.72 ± 14.51 10% 449 1156 0.8931 2.954 39.23 39.5 201.01 ± 27.85158.19 ± 22.43 12% 496 1194 2.019 6.822 37.24 34.1 356.96 ± 15.96 193.98± 48.14 15% 515 1230 8.372 20.64 34.61 29.71  478.9 ± 18.37 297.03 ±14.74

While SDS increased both the conductivity and the viscosity of thesolutions significantly, it had surprisingly litte effect on the surfacetension. The diameters of the nanofibers were decreased by adding 1 wt %SDS to various concentrations of PVA. At the lowest PVA concentration(6.5 wt %), the fiber morphology changed from bead-on-string (with noSDS) to uniform fibers on the addition of 1 wt % SDS.

Example 3 Electrospinning Effects on Nanofiber Characteristics

In order to further characterize PEO/magnetite and PVA/magnetitenanofibers, transmission electron microscropy was utilized to visualizethe fibers (FIG. 6). The weight percentages of magnetite nanoparticleswithin the fibers were 28%, and 8% for PEO/magnetite and PVA/magnetitenanofibers, respectively. The relatively large size of the PEO fiber andhigh content of nanoparticles within the fiber made it difficult tofocus the TEM pictures, but the contour of the alignment of thenanoparticles into columns along the fiber axis direction was readilyvisible. For the PVA/magnetite fiber, the images were clearer, anddemonstrated magnetite nanoparticle alignment in columns parallel to thefiber axis direction within the fiber.

Magnetite nanoparticles can form chains in solution owing to magneticcoupling effects between particles. The number of nanoparticles, n_(0.)in a chain in the fluid, at zero external field, can be estimated usingthe following formula: $\begin{matrix}{n_{0} = \left\lbrack {1 - {\frac{2}{3}\left( \frac{\phi}{\lambda^{3}} \right){\mathbb{e}}^{2\lambda}}} \right\rbrack^{- 1}} & (1)\end{matrix}$where, φ is the volume fraction of particles in the fluid, and λ is thecoupling coefficient, which measures the strength of particle-particleinteractions.λ is given by $\begin{matrix}{\lambda = \frac{\mu_{0}M^{2}V}{14{kT}}} & (2)\end{matrix}$where, μ₀ is the permeability of free space, M is intensity ofmagnetization of the magnetic particles, V is the volume of the magneticparticles, k is Boltzmann's constant, and T is the absolute temperaturein degrees Kelvin.

For magnetite particles 7.5 nm in diameter, λ<1 according to equation(2), which means the particle-particle interaction energy is less thanthe thermal energy and no chain forms in solution prior toelectrospinning for any concentrations of magnetite particles Thus thecolumn alignment of magnetite nanoparticles within the fiber observed inFIG. 6(b) was a result of the electrospinning process itself. Possiblecauses for this alignment may have been hydrodynamics in the capillary,steady jet, or whipping jet regions, or induction by the local electricfield.

Super Conducting Quantum Interference Device (SQUID) magnetizationcurves for both 28% PEO/magnetite and 8% PVA/magnetite nanofibersdemonstrated the superparamagnetic behavior of the fibers at roomtemperature (FIG. 7). At low temperature (5K), both systems werecharacterized by a narrow hysteresis and a small remnant magnetizationat zero field. These can be explained by considering the magneticrelaxation of the nanoparticles. For 7.5 nm diameter particles, Neelrelaxation dominates the Brownian rotation mechanism. The Neelrelaxation time varies exponentially with inverse temperature. Forexample, at 300 K the Neel relaxation time for magnetite particles 8 nmin diameter in kerosene carrier is approximately 10⁻⁹ s, and increasesto approximately 13 s at 5K. At low temperature, when the applied fieldreached zero, the dipole moments of some nanoparticles were stillpolarized, and therefore a small remnant magnetization was observed. Thesuperparamagnetic behavior of the nanofibers at room temperature may beuseful, in some embodiments of this invention, for applications in whichalternating nonuniform fields are needed, as this would reduce thedissipative energy in a device comprising the nanofibers.

Example 4 Structural Characterization of the Composite NanofibersComprising Magnetite Nanoparticles

The elastic modulus of the fibers was evaluated using an AFM indentationtechnique according to the following formula [Vanlandingham M. R., etal., J. Adhesion 1997; 64: 31-57; Sneddon J. N., Int. J. Engng. Sci.1965; 3: 47-56; Pharr G. M., et al., J. Mater. Res. 1992; 7: 613-617;Vanlandingham M. R., et al., Composites Part A1999; 30:75-83; DrechslerD., et al., Appl. Phys. A 1998; 66: S825-S829]: $\begin{matrix}{S = {{\frac{\mathbb{d}P}{{\mathbb{d}\Delta}\quad Z_{i}}❘_{P_{\max}}} = {2{E^{*}\left( \frac{A}{\pi} \right)}^{\frac{1}{2}}}}} & (3)\end{matrix}$

Here, S was the slope of the unloading curve at P_(max), P was theapplied load, A was the contact area, ΔZ_(i) is the indentation depth,and E* was the effective Young's modulus of the contact as defined by$\begin{matrix}{\frac{1}{E^{*}} = {\frac{1 - v_{s}^{2}}{E_{s}} + \frac{1 - v_{t}^{2}}{E_{t}}}} & (4)\end{matrix}$

In equation (4), E_(s) and E_(t) were the elastic moduli, and v_(σ) andv_(t) the Poisson ratios of the sample and the tip, respectively. Adiamond tip was used, with asymmetric pyramidal geometry; indent sizewas characterized by the lateral distance from the apex to the base ofthe triangular impression [Vanlandingham, supra]. E_(t) and v_(t) wereassumed to be 130 GPa and 0.2, respectively, corresponding to the bulkvalues of diamond [Kracke B., Damaschke B. Appl. Phys. Lett. 2000;77:361-363; Vanlandingham M. R., et al., J. Mater. Sci. Lett. 1997;16:117-119; Klapperich C., et al. J. Tribology 2001; 123: 624-631]. Thenanofibers were indented in the radial direction. A schematic depictionof the tip-sample interaction during the indentation test is shown inFIG. 8. The method was applicable here, in view of the fact that thediameters of the fibers (>150 nm) were much larger than the diameters ofthe contact area (<10 nm).

The results of the indentation tests for PVA/magnetite nanofiber areshown in FIG. 9. Mica was also indented to evaluate the bending of theAFM cantilever, which was then subtracted from the raw PVA/magnetitedata to determine fiber properties. In these indentation tests, theslopes of the top portions of the unloading curves were used to evaluatethe modulus of each sample.

Assuming the tip geometry is the same for all the indentations, therelative changes in indent size was sufficient to relate contact areas;here, the apex to base distance was equated to a contact radius, r. ThePoisson ratio, v, was assumed to be the same for all fibers within eachgroup. The ratio of modulus of different samples within each group wasthen evaluated using the formula: $\begin{matrix}{\frac{\left( {{{\mathbb{d}P}/{\mathbb{d}\Delta}}\quad Z_{i}} \right)❘_{P_{\max}^{1}}}{\left( {{{\mathbb{d}P}/{\mathbb{d}\Delta}}\quad Z_{i}} \right)❘_{P_{\max}^{2}}} = \frac{r_{1}E_{1}}{r_{2}E_{2}}} & (5)\end{matrix}$

Table 3 provides the indentation data obtained for the nanofibers. TABLE3 Nanoindentation data of the nanofibers. PVA¹ PVA + Fe₃O₄ ¹ PEO² PEO +Fe₃O₄ ² Epoxy¹ Epoxy² ΔZ_(i) (nm) 12.33 ± 4.46 14.55 ± 2.67 9.36 ± 1.415.92 ± 0.68 22.14 ± 1.42 10.92 ± 0.35 ΔZ_(i) (nm) 80.04 ± 5.05 51.23 ±4.46 170.32 ± 15.65  194.24 ± 14.40  67.83 ± 1.42 38.94 ± 1.87 Modulus 4.8 ± 1.73  4.1 ± 0.75 0.66 ± 0.10 1.04 ± 0.12  1.52 ± 0.12*  1.52 ±0.12* (GPa)¹Trigger setpoint of deflection signal of the cantilever is 1.2 V.²Trigger setpoint of delfection signal of the cantilever is 0.6 V.*The modulus is determined independently by a triboindenter using aBerkovich tip.

ΔZ; represented the total displacement recovered from P=P_(max) to P=0.ΔZ_(t) represented the total indentation depth, which measures thepenetration of the tip into the sample surface, including both theinelastic and elastic deformation of the material. The moduli wereobtained by comparing values obtained with reference epoxy sample withineach group, using equation (5). The modulus of the reference epoxysample was determined to be 1.52 GPa using a Triboindenter. The indentsize, r, was found to be 40 nm for the PEO and PEO/magnetite nanofibers,10 nm for PVA and PVA/magnetite nanofibers, 20 nm and 10 nm forreference epoxy samples under conditions used to test the PVA group andPEO group, respectively.

As shown in Table 3, after including magnetite nanoparticles (8 wt %)within PVA nanofibers, ΔZ_(i) was statistically the same, and ΔZ_(t)decreased. This indicates that the modulus of the PVA nanofibers wasmaintained and the inelastic deformation was decreased due to thereinforcement effect of magnetite nanoparticles.

After including (28 wt %) magnetite nanoparticles within the PEO fiber,ΔZ_(i) decreased, but ΔZ_(t) increased showing that the modulus of PEOnanofibers was increased due to the reinforcement effect of magnetitenanoparticles. One indication of the increase in inelastic deformationof the PEO/magnetite nanofibers was that the short chains of polymericshell around the nanoparticles may have been detrimental to themechanical properties of the nanofibers, and overwhelmed the effect ofmagnetite reinforcements as the concentration of the magnetitenanoparticles within the fibers increased from 8 wt % to 28 wt %.

The superparamagnetic fibers exhibited mechanical properties comparableto those of the polymer matrix, and were not brittle. Though the fiberswere ceramic, and therefore expected to be very brittle, and unlikely todeform at all, much less to as large a strain as produced int thesuperparamagnetic fibers.

Example 5 Composite Nanofibers Comprising Magnetite Nanoparticles AreSuperparamagnetic

SQUID tests demonstrated that the magnetite nanoparticles within thenanofibers were easily magnetized by an external magnetic field, andthat the dipole moments of the nanoparticles were readily polarized inthe direction of the external magnetic field.

Since a magnetic dipole experiences a torque force in a uniform magneticfield and a translational force in a magnetic field gradient, it wasthought that the composite nanofibers containing magnetite particles inan external magnetic field gradient, may be deformed by thetranslational forces experienced by the embedded nanoparticles (FIG.10). In response to an electric field provided by a small laboratorymagnet, a strip of PVA/magnetite nonwoven mat exhibited field-responsivebehavior.

In this example, one end of the nonwoven mat was fastened onto thesurface of a table, while the other end was free to move. In the absenceof the magnetic field, the nonwoven mat lay flat on the surface of thetable (FIG. 10 a). When the magnet was placed above the nonwoven mat,the fabric was deflected by the translational forces in the direction ofincreasing magnetic field as shown in FIG. 10 b. As the magnet wasbrought closer to the fabric, the greater magnetic field gradientsexperienced by the nonwoven mat induced larger translational forces onthe magnetite nanoparticles, causing a greater deflection of the freeend of the mat towards the magnet (FIG. 10 c). The PEO/magnetitenonwoven mat showed similar response behavior to the laboratory magnetas the PVA/magnetite nonwoven mat. Thus, both superparamagnetic fabricsproduced by the electrospinning techniques exemplified herein, exhibitedfield-responsive behavior.

Superparamagnetic polymer nanofibers ranging in diameter from 140 to 400nm obtained via the electrospinning of polymer-stabilized magnetitenanoparticle suspensions in PEO and PVA solutions exhibited nanoparticleline up within the fibers in columns parallel to the fiber axisdirection. Both sets of fibers were superparamagnetic at roomtemperature, and responded to an externally-applied magnetic field bydeflecting in the direction of increasing field gradient, withnanoindentation tests -showing magnetite nanoparticle reinforcement ofthe mechanical properties of nanofibers.

Example 6 Composite Nanofibers Comprising Organic-Soluble PolymersMaterials and Methods

Reagents

Iron(III) acetylacetonate (97%), Benzyl ether (99%), 1-2 hexadecanediol(97%), ethanol, oleic acid (90%) and oleylamine (70%) were purchasedfrom Sigma Aldrich and used as received.

Synthesis of seeds

2 mmol of Iron (III) acetylacetonate, 10 mmol of 1-2 hexadecanediol, 6mmol of oleic acid and 6 mmol of oleylamine and 20 ml of benzyl etherwere mixed in a 3 neck flask and were stirred continuously under ablanket of nitrogen. The temperature was ramped up slowly to 200° C.(2.5° C./min) and the mixture was kept at this temperature for 2 hrs.Finally the mixture was refluxed at 300° C. for 1 hr. The resultingblack mixture was cooled down to room temperature and ethanol was addedfollowed by centrifugation at 7000 g to separate out magnetite. Thecentrifuged product was re-suspended in hexane and was used for seedmediated growth.

Synthesis of Bigger Nanoparticles

2 mmol of Iron(III) acetylacetonate, 10 mmol of 1-2 hexadecanediol, 2mmol of oleic acid and 2 mmol of oleylamine, 20 ml of benzyl ether and80 mg of seeds in 4 ml of hexane were mixed in a 3 neck flask andstirred continuously under a blanket of nitrogen. The mixture was keptat 100° C. for 30 mins and at 200° C. for 1 h. Finally the mixture wasrefluxed at 300° C. for 30 mins. The magnetite was recovered using theprocedure outlined above. The resulting magnetite nanoparticles can thenbe used as seeds for subsequent synthesis. In this manner stablenanoparticles up to 16 nm were synthesized.

Electrospinning

First, PMMA was directly added into DMF solvent to make 7.5 wt %solution. PMMA was directly added into THF suspension containing 3.7 wt% of Fe₃O₄. Mixing the two solutions prepared above at a ratio of 3:1, 17.5 wt % PMMA solution containing 2.78 wt % Fe₃O₄ in a mixture of THFand DMF (3:1) was prepared for electrospinning. A parallel-plateelectrospinning apparatus was used in this research and has beendescribed elsewhere by Shin et al [Appl. Phys. Lett. 78, 1149-1151(2001)] and Fridrikh et al [Phys. Rev. Lett. 90:144502 (2003)].

Measurement and Characterization

The images of the electrospun fiber were obtained using a JEOL-6060SEM(JEOL Ltd, Japan), and the fiber diameters were determined usingAnalySIS image processing software (Soft Imaging System Corp., Lakewood,USA) by measuring 20 randomly selected fibers for each sample.Transmission Electron Microscopy (TEM) was performed on the JEOL JEM200CX TEM microscope (JEOL Ltd, Japan). The Superconducting QuantumInterference Device (SQUID) test was conducted using an MPMS XLmagnetometer (Quantum Design Inc., San Diego) for PMMA/magnetitenanofibers.

RESULTS

In order to determine whether nanoparticles could be prepared via anorganic route, synthesis using organic solvents was undertaken. The TEMmicrographs of magnetite nanoparticles of different diameterssynthesized by the organic route are shown in FIG. 11. The TEMmicrographs confirmed the seed mediated growth. It was difficult tore-suspend bigger nanoparticles in hexane (>14 nm), possibly due tohigher magnetic interaction between the bigger nanoparticles.Nanoparticles were sonicated to get a suspension in THF. The sonicationtended to disturb the alignment, thereby aiding in dispersion. The shapeof the nanoparticles changed from spherical to cubical shape withincreased particle size, perhaps attributable to a different amount ofsurfactants adsorbing on different faces of the growing crystal.

Electrospinning of the PMMA/16 nm magnetite nanoparticle dispersion inTHF was not possible as the jet dried too fast at the capillary tip. Thejet became stable, however, following the addition of a 33% in volume ofDMF in THF dispersion, and a uniform fiber was obtained.

The preferred processing parameters were 17.5 kV for applied electricalpotential, 0.05 ml/min for flow rate, and 25 cm for the plate-to-platedistance. The current measured was 72.5 nA and the diameter of the fiberwas around 1.53±0.34 μm. A representative SEM picture of the electrospunPMMA fiber containing 16 nm magnetite nanoparticles is shown in FIG. 12.

A SQUID magnetization curve for PMMA fiber containing 37% by weight of16 nm magnetite particles is shown in FIG. 13. At room temperature, asmall hysteresis and a small remnant magnetization at zero field areobserved. This is in contrast to the superparamagnetic behavior observedfor the magnetic fibers containing 7.5 nm magnetite nanoparticles, shownabove. These can be explained by considering the magnetic relaxation ofthe nanoparticles. For the particles within the fiber, only the Néelrelaxation mechanism is operative, since the nanoparticles can notrotate through a Brownian mechanism within the fiber matrix. 16 nmmagnetite nanoparticles have a longer Néel relation time than 7.5 nmmagnetite nanoparticles. At room temperature, when the applied fieldreached zero, the dipole moments of the 16 nm nanoparticles were stillpartially aligned while those of the 7.5 nm nanoparticles werecompletely relaxed, and therefore a small remnant magnetization wasobserved for fibers containing 16 nm nanoparticles, but not forcontaining 7.5 nm nanoparticles.

In this example, textiles comprised of magnetic fibers containing 16 nmmagnetite nanoparticles have been produced by electrospinning. Theorganic synthesis route for nanoparticles employed here complements theaqueous synthesis route above, and provides for functionalization of thenanoparticles with an organic surface coating that is compatible withorganic solvents. The resulting particles can therefore be dispersed ineither aqueous or organic solutions, respectively. The range of polymersthat can be electrospun with magnetic particles to form field-responsivefibers is thus expanded to include both organic-soluble andwater-soluble polymers.

The organic route synthesis, in contrast to the aqueous route presentedabove also permits a wider range of particle compositions, includesthose with larger intrinsic magnetic moments. The larger particlesproduced via organic route result in a longer Néel relaxation time, asdemonstrated by the SQUID test, where the remnant magnetization at zerofield for the 16 nm particles indicated ferromagnetic behavior, ratherthan the superparamagnetic behavior exhibited by the smaller particlesobtained via aqueous synthesis.

The longer Néel relaxation time provides materials, which exhibitchanges in mechanical properties under a uniform applied field atconventional rates of deformation. Due to coupling of the particlemagnetic moment with the applied field, deformation of the magneticfibers requires additional work, resulting in increased stiffness andlower strain, compared to the equivalent nonmagnetic fiber at equaldeformation energy.

1. A superparamagnetic fiber comprising magnetite particles and apolymeric matrix.
 2. The superparamagnetic fiber of claim 1, whereinsaid fiber is a nanofiber.
 3. The superparamagnetic fiber of claim 2,wherein said nanofiber is less than 500 nm in diameter.
 4. Thesuperparamagnetic fiber of claim 2, wherein said nanofiber diameterranges -from 10 nm-1 μm.
 5. The superparamagnetic fiber of claim 1,wherein said matrix comprises polyethylene oxide, polyvinyl alcohol or acombination thereof.
 6. The superparamagnetic fiber of claim 1, whereinsaid polymeric matrix comprises a polysaccharide, an oligosaccharide, asurfactant, a polyethylene glycol, a lignosulfonate, a polyacrylamide, apolypropylene oxide, a cellulose derivative a polyacrylic acid or acombination thereof.
 7. The superparamagnetic fiber of claim 1, whereinsaid polymeric matrix comprises any polymer that can be electrospun fromsolution.
 8. The superparamagnetic fiber of claim 1, wherein saidsuperparamagnetic fiber is magnetic field-responsive.
 9. Thesuperparamagnetic fiber of claim 1, wherein said magnetite nanoparticlesare stably dispersed within said polymeric matrix.
 10. A device orapparatus comprising the superparamagnetic fiber of claim
 1. 11. Thedevice or apparatus of claim 10, wherein said device or apparatus isused as a filter or a sensor.
 12. The device or apparatus of claim 10,wherein said device or apparatus is used for information storage. 13.The device or apparatus of claim 10, wherein said device or apparatus isused for magnetic imaging.
 14. The device or apparatus of claim 10,wherein said device or apparatus is used for magnetic shielding.
 15. Thedevice or apparatus of claim 10, wherein said device or apparatus isused as a tunable mechanical reinforcement component in a composite 16.The device or apparatus of claim 10, wherein said device or apparatus isused as a piezomagnetic transducer
 17. A fabric comprising thesuperparamagnetic fiber of claim 1, wherein said fabric may be woven ornonwoven.
 18. A field-responsive fiber comprising ferromagneticnanoparticles and an organic polymeric matrix.
 19. The fiber of claim18, wherein said fiber is a nanofiber.
 20. The fiber of claim 19,wherein said nanofiber has a diameter ranging from 10-500 nm.
 21. Thefiber of claim 18, wherein said matrix comprises polymethylmethacrylate.
 22. The fiber of claim 18, wherein said nanoparticles aremonodispersed within said polymeric matrix.
 23. The fiber of claim 18,wherein said fiber has a high saturation magnetization, ranging from 250kA/m to 2000 kA/m.
 24. The fiber of claim 18, wherein said fiber has atunable Nèel relaxation time which ranges from 2 milliseconds to 4seconds.
 25. The fiber of claim 24, wherein said tunable Nèel relaxationtime is a function of nanoparticle size.
 26. A device, apparatus orfabric comprising the fiber of claim
 18. 27. A method of producing afield-responsive fiber comprising magnetite particles and a polymericmatrix, the method comprising the step of electrospinning a polymersolution comprising magnetic nanoarticles.
 28. The method of claim 27,wherein said field-responsive fiber is a nanofiber.
 29. The method ofclaim 27, wherein said nanofiber is less than 500 nm in diameter. 30.The method of claim 29, wherein said nanofiber diameter ranges from 10nm-1 μm.
 31. The method of claim 27, wherein said field-responsive fiberis superparamagnetic.
 32. The method of claim 31, wherein said polymersolution comprises polyethylene oxide.
 33. The method of claim 32,wherein said polyethylene oxide is at a concentration of between 1% and3% by weight.
 34. The method of claim 33, wherein said polymer solutionhas a conductivity of between 0.1 and 10000 μS/cm.
 35. The method ofclaim 31, wherein said polymer solution comprises polyvinyl alcohol. 36.The method of claim 35, wherein said polyvinyl alcohol is at aconcentration of between 6.5% and 15% by weight.
 37. The method of claim31, wherein said polymer solution comprises SDS.
 38. The method of claim31, wherein said polymer solution comprises a polysaccharide, anoligosaccharide, a surfactant, a polyethylene glycol, a lignosulfonate,a polyacrylamide, a polypropylene oxide, a cellulose derivative apolyacrylic acid or a combination thereof.
 39. The method of claim 31,wherein said polymer solution is an aqueous solution.
 40. The method ofclaim 27, wherein said field-responsive fiber is ferromagnetic.
 41. Themethod of claim 40, wherein said matrix comprises polymethylmethacrylate.
 42. The method of claim 40, wherein said nanoparticles aremonodispersed within said polymeric matrix.
 43. The method of claim 40,wherein said fiber has a high saturation magnetization, ranging from 250kA/m to 2000 kA/m.
 44. The method of claim 40, wherein said fiber has atunable Nèel relaxation time which ranges from 2 milliseconds to 4seconds.
 45. The method of claim 44, wherein said tunable Nèelrelaxation time is a function of nanoparticle size.
 46. The method ofclaim 40, wherein said polymer solution is an organic solution.
 47. Themethod of claim 27, wherein said polymer solution comprises any polymerthat can be electrospun from solution.
 48. A superparamagnetic fiberproduced by the method of claim 27.